Developer Notes

Without proper kernel setting that allows RNDIS via USB connection (CONFIG_USB_ETH with CONFIG_USB_ETH_RNDIS, or CONFIG_USB_G_ANDROID), device did seem to hang at the initial boot screen while displaying "
KERNEL IS NOT SEANDROID ENFORCING
Set Warranty Bit : kernel

SAMSUNG
Galaxy Tab S2

POWEREd BY
android
"
but this was only due to not having an RNDIS connection via USB!

Not mentioned in the porting docu yet: Had to use uppercase partition labels in deviceinfo_flash_heimdall_partition_(kernel|system) variables in deviceinfo file, even if their symlinks are lowercase (e.g. "system -> /dev/block/mmcblk0p36"), otherwise get 'ERROR: Partition "system" does not exist in the specified PIT.'

libinput from git master - touchscreen works partially

compiled libinput from git master, using commit 2167c3af058aebf5ce732ed0a3a1c5488b2a9d73, from https://github.com/wayland-project/libinput - touchscreen behaviour is better now, the _false_ palm detection is no longer all the time active, as the configured ENV{LIBINPUT_ATTR_PALM_PRESSURE_THRESHOLD}=256 is now applied.
commit 59eb10e593213403c301a5ce3fbd0a22b6e52efd 'util: allow for palm pressure > 255' or newer is needed for that to work.
With this new libinput version, the mouse pointer from plasma-mobile can be moved and clicking also does work as expected. Not yet working: 90-degree left rotation is not applied.